The present invention relates to tappet in use for a direct acting type valve operating mechanism in an internal combustion engine.
In FIG. 7, "A" denotes a conventional tappet in use for a direct acting type valve operating mechanism in an internal combustion engine. The tappet "A" comprises a cylinder closed by a top wall 22 at the upper end, and in the middle of the lower surface of the top wall 22, there is integrally formed a projection 23 which contacts the end of a stem of an engine valve (not shown). The tappet "A" is generally molded by cold forging of low carbon mild steel, and is then subjected to heat treatment such as cementation and hardening.
However, when the conventional tappet "A" as above is subjected to heat treatment such as cementation and hardening, thermal expansion occurs, thereby decreasing accuracy in size of each portion, which involves high cost for mechanically cutting and processing. Further, it requires a number of mechanical processing steps which is troublesome, thereby decreasing productivity.
In the top wall of the tappet "A" which is molded from mild steel, there is low strength at the lower surface which contacts the end of the stem of the engine valve and at the upper surface which contacts a rotary cam even though each portion is strengthened with cementation and hardening. Thus, compression load repeatedly acts up and down to each portion, thereby deforming it locally and increasing wear rate.